One second in 30 billion years. That is the error margin in the most precise atomic clock that humankind has produced today. Atomic clocks rely on stable atomic transitions as frequency references, but this method doesn’t come without problems. Trapped-ion atomic clocks are sensitive to stray electric fields which cause the ion to move and experience Doppler shifts, and this decreases the clock’s precision and accuracy.
It is this problem that Gerard Higgins, researcher at Chalmers University of Technology, has found a new solution to. He demonstrated the technique with Markus Hennrich’s Trapped Ion Quantum Technology group at Stockholm University.
“I came up with a technique to more precisely measure unwanted electric forces acting on a trapped ion, and I demonstrated it experimentally”, he says. “My technique allows the forces to be measured more quickly and more precisely than the existing techniques.”
Drives the development of quantum physics
Atomic clocks go hand in hand with precision spectroscopy. In precision spectroscopy, the energy levels of atoms and molecules are probed, and used to reveal properties of atoms and molecules, and their constituent electrons, protons and neutrons. Spectroscopy has driven the development of quantum physics – as measurements have become more precise, unexpected results have been found and quantum theory has had to be refined. Likewise, spectroscopy has allowed new theories to be tested, and continues to be used to search for new physics; researchers are using ever more precise spectroscopy to test whether the fundamental constants are really constant, and to test the similarities and differences between normal matter and anti-matter.
“Ever more precise spectroscopy requires ever more careful control of unwanted effects which can bias the results, such as unwanted electric fields in an ion trap”, says Gerard Higgins.
The new technique will make atomic clocks more accurate, as they involve spectroscopy. They use the difference between two atomic energy levels as a frequency reference, and so more precise spectroscopy means more precise atomic clocks.
Unwanted electric fields can also limit the fidelity of a trapped ion quantum information processor, the sensitivity of a trapped ion force sensor, and pose limits to fundamental studies of trapped ion-neutral atom interactions. With the new technique, unwanted electric fields can be probed and compensated, so that they don’t pose a problem.
“To my knowledge, the technique is faster than the existing techniques – this means one doesn’t need to spend much time interrupting the main experiment to be able to diminish unwanted fields, and the technique is more precise than the existing techniques. What’s more it’s quite easy to implement and automate”, says Gerard Higgins.
About the new technique
The new technique uses interferometry to precisely measure the ion displacement caused by the weak forces. Interferometers are used for the most sensitive displacement measurements, for instance most gravitational wave detectors are interferometers. Instead of a normal interferometer with two optical paths, though, the new technique relies on a technique called Ramsey interferometry. Ramsay interferometry is a standard technique used in experimental quantum physics, often used for quantifying a qubit’s coherence time. During Ramsey interferometry, the trapped ion qubit is excited to a superposition of two electronic states, and two laser pulses act as an interferometer’s beam splitters. The ion qubit is sensitive to the laser phase, so if the ion is displaced between application of the laser pulses, a measurement of the qubit reveals the displacement
Read the full scientific article in New Journal of Physics
Postdoc at the Department of Microtechnology and Nanoscience, Quantum Technology Laboratory
Text: Robert Karlsson
Photos: Markus Hennrich and Cristine Calil Kores
Illustration: Gerard Higgins, Marion Mallweger and Robin Thomm